Abstract
Several authors have investigated theoretically and experimentally the signal/noise ratio in receiving aerial systems, but the problem of the validity of Nyquist's theorem for radiation resistance has hitherto not been satisfactorily resolved.
The problem is here discussed, and it is concluded that (i) for an aerial in an enclosure at uniform temperature, the radiation resistance is at that temperature from the point of view of noise, and thus (ii), for an aerial in free space, no noise originates in the radiation resistance. These results are shown to be consistent with the Rayleigh-Jeans radiation law, while the quantum-theory form of Nyquist's equation leads to the Planck radiation law.
The estimation of the noise occurring in practical aerial systems is discussed, consideration being given to the various external noise fields, viz. thermal radiation, Jansky noise, atmospherics. The actual noise received may be expressed conveniently by the equivalent temperature Tr of the radiation resistance. The general problem of evaluating the signal/noise ratio for any values of received circuit- and valve-noise is analysed and a criterion (K) of efficiency of an aerial system is deduced. The paper concludes with a numerical calculation of the performance of (1) a vertical aerial inductively coupled to a tuned-grid circuit and (2) a tuned-loop aerial, which are typical examples of an efficient and an inefficient system respectively.